The present invention relates to a scintillation camera apparatus for obtaining a scintigram of a living body into which a gamma-emitting radio isotope (RI) is injected.
Medical equipment which utilizes a scintillation camera is popular. In this type of equipment, the distribution of gamma rays radiated from an object to be examined, into which a radio isotope is applied, is detected for diagnosis of the object. A scintillation camera comprises a collimator, a scintillator, and photomultipliers arranged in a matrix form. Gamma rays radiated from an object to be examined bombard the scintillator through the collimator to generate fluorescent light. The fluorescent light is incident on the respective photomultipliers which produce electrical signals proportional to the intensity of the incident gamma rays. The electrical signals are supplied to an electronic circuit comprising a position calculation circuit, an addition circuit, and the like, thus generating position signals X and Y representing the detection position of the gamma rays and an energy signal Z representing the intensity of the gamma rays. The detection frequency of the energy signal Z whose magnitude is within a certain range (window) is stored in an image memory for each detection position, and data in the image memory is read out to a display device having a cathode ray tube (CRT) display to display the scintigram of the object. Due to its structural limitations, the scintillation camera is inevitably subject to errors caused by the nonlinearity of radiation detection positions and energy detection errors caused by variations in gains of the photomultipliers. For this reason, at the manufacturing stage of the scintillation camera apparatus, it is essential to compensate for the nonlinearity of the detection positions (i.e., position signals X and Y) and variations in energy detection response (i.e., energy signal Z), in order to obtain accurate scintigrams.
For example, the characteristics of the scintillation camera are measured using a uniform radiation source to prepare a nonlinearity compensation table (i.e., an X-Y compensation table) which stores compensation vectors for compensating for the nonlinearity of the detection positions and an energy detection sensitivity compensation table (i.e., a Z compensation table) for compensating for variations in energy detection response at pixel positions of the camera. In actual diagnosis of the object, position signals X and Y and energy signal Z obtained from the scintillation camera are corrected by compensation data read out from the correction tables comprised of random access memories (RAMs) accessed by these signals.
However, a change in gain characteristics of the photomultipliers with passage of time (i.e., a change in peak values of Z signal) is inevitable. When such a change with time in gain occurs, the initially prepared Z compensation table becomes useless. For this reason, an operator is required to update the Z compensation table at intervals of a constant time in accordance with the current gain characteristics of the photomultipliers. This operation is complicated and time consuming.